This invention relates to a wet abrasive blasting procedure for cleaning hard surfaces, such as steel or concrete structures. In particular, the invention provides a nozzle unit designed especially for using wet abrasive materials in cleaning operations.
Dry abrasive blasting is a technique that has been used for many years to remove rust, scale, old paint, etc. from steel structures, such as pipelines, highway bridges, storage tanks, and from other hard surfaces, such as brick and concrete. A common abrasive material used in this cleaning operation is a standard grade of silica sand. During such an operation the free silica creates a significant amount of dust in the atmosphere near the surface being blasted.
Since the silica dust pollutes the environment, many states have enacted laws in the last few years that restrict the amount of abrasive material that can be released into the atmosphere. One attempt to solve the dust problem is wet abrasive blasting. This method is now widely used in many industrial cleaning operations, because it suppresses a considerable amount of the dust usually generated in a dry blasting operation.
The "water shroud" method is one form of a wet abrasive blasting operation. This method involves attaching a "water" ring to the outer tip of a conventional, long venturi blast nozzle. As the air-sand stream exits the nozzle, water is pumped through holes in the ring, so that it impinges on this stream and "wests down" the sand.
The water shroud method has several drawbacks. For example, it uses excessive amounts of water, because the blast stream (air and sand) tends to blow the water out of its path as it exits the blast nozzle. And the more water that is injected into the blast stream, the more it reduces the velocity of the sand and air. This results in a lower production rate, because it takes longer to complete a given job.
Another type of wet abrasive blasting is the "water injection" method. In this method, water is injected into the blast stream before it enters the blast nozzle. The water is injected at a pressure above that of the line pressure of the blast stream (about 100 psi), so that it can mix well with the sand. This method also uses large amounts of water, and it requires a pump capable of exceeding the line pressure.
Another wet abrasive blasting method used very high pressure water (from about 600-20,000 psi) as the primary force. This system employs a special nozzle head that creates negative pressure induced by the venturi structure of a conventional blast nozzle. The sand abrasive is carried through a suction hose and mixes with the water stream before the water and sand enter the nozzle bore. This system also uses large amounts of water, and the velocity of the blast stream is too low for good abrasive impingement.